Conventional polyether and polyester polyurethane foams are hydrophobic: they do not readily absorb liquids, such as water. Certain foam applications require foams that will absorb water. Such foams are known as hydrophilic.
Hydrophilic polyurethane foams can be prepared by a "prepolymer" process in which a hydrophilic prepolymer having isocyanate end groups is mixed and reacted with water. U.S. Pat. Nos. 3,861,993 and 3,889,417 disclose a hydrophilic polyurethane foam which is formed by mixing and reacting with water an isocyanate capped polyoxyethylene glycol prepolymer using a molar ratio of H.sub.2 O to NCO groups in the prepolymer of 6.5 to 390:1. Commercial hydrophilic polyurethane foams of this type, known as HYPOL.RTM. foams, are prepared by mixing and reacting the prepolymers with water. HYPOL.RTM. foam prepolymers are available from Hampshire Chemical Company.
A particular family of polyurethane prepolymers, derived from methylene diphenyl diisocyanate (MDI) and sold under the trademark HYPOL PLUS, was developed by W. R. Grace & Company and is now sold by Hampshire Chemical Company for use in an aqueous two-stage process of foam production. The prepolymers, and the aqueous two-stage process foams produced therefrom, are disclosed in U.S. Pat. No. 4,365,025. An isocyanate-containing prepolymer in which the isocyanate is a mixture of MDI and polymeric forms of MDI is foamed by mixing it with an approximately equal amount of water. The resultant flexible foams are characterized by greater hydrolytic stability than those foamed from toluene diisocyanate (TDI) prepolymers.
Wet out is generally the time interval it takes for a liquid to penetrate the foam surface. For many applications, the rate at which the foam picks up the liquids that contact the foam surface is critical. Wet out rates vary greatly for foams characterized as hydrophilic. Most hydrophilic foams do not display rapid wet out. Often other hydrophilic materials, such as fibers (See, e.g., U.S. Pat. No. 4,127,516) or superabsorbent polymers (See, e.g., U.S. Pat. No. 5,064,653) or particles (See, e.g., U.S. Pat. No. 3,224,889), must be incorporated into the HYPOL.RTM. foam structures to improve the wet out rates of such foams.
Other hydrophilic polyurethane foams have been disclosed in the art. Very few concern producing foams with improved wet out rates.
U.S. Pat. No. 3,961,629 discloses a hydrophilic polyurethane foam in which the foam pores have an applied surfactant outer coating to accelerate absorption of body fluids into the pores at medically acceptable rates. The term "medically acceptable rate" is not defined as a range of rates in the patent, although the examples show blood absorption rates of 2 to 60 seconds.
U.S. Pat. No. 4,314,034 discloses a polyurethane foamed sponge formed from the combination of a hydrophilic oxyalkylene polyol capped with isocyanate groups and 1 to 30% by weight of a polymeric polyisocyanate, such as a PAPI resin, a commercially available form of methylene diphenyl isocyanate. The prepolymers are foamed in the presence of water, reinforcing fibers, surfactants, a thickening agent and up to 40% by weight of diatomaceous earth. With the diatomaceous earth and PAPI the foam sponge has what is described as rapid wet out and improved wet strength.
U.S. Pat. No. 4,740,528 discloses an absorbent "superwicking" crosslinked polyurethane foam that has an effective amount of at least one amino acid incorporated into the foam-forming composition. The patent suggests that surfactants may be added to the foam forming composition, but that surfactants are not known to significantly contribute to the wicking properties of the foam (Col. 10, line 31).
In U.S. Pat. No. 4,191,815, hydrophilic polyurethane foams are prepared by reacting a polyol with a polyisocyanate that has NCO groups that are not bonded to an aromatic ring in the presence of effective amounts of certain catalysts, foam stabilizers and swelling agents. The resulting foams have reported wet out rates or penetrability rates generally over 30 seconds.
Surface-active materials, i.e., surfactants, are generally added to foam-forming compositions to help control the size and shape of the foam cells by stabilizing the gas bubbles formed during nucleation. Surfactants also help to control the degree of cell opening and increase the operating margin between the extremes of shrinkage or reduced permeability and foam collapse, when cell opening occurs before the reaction mixture has sufficiently polymerized beyond its "gel point." Because some surfactants are known to leach out of foams when the foams absorb liquids, some hydrophilic foams are made without addition of any surfactants (See, e.g., U.S. Pat. No. 5,296,518).
Silicone-polyether liquid copolymer surfactants are known to produce foams with small, fine cells (See, e.g., U.S. Pat. No. 5,104,909 at Col. 4, lines 1-14). It was not expected that when certain of these silicone glycol copolymer liquid surfactants were incorporated into hydrophilic foam-forming compositions, the resulting foams would have more rapid wet out.
One object of the present invention is to produce hydrophilic polyurethane foams that do not require hydrophilic fibers or other added materials foreign to the foam to increase the wet out rate of such foams.